1. Field of the Invention
The present invention relates to an improvement on a motor and/or a generator, and more particularly to the reduction of loss in current in the coils by utilizing a superconducting material in the coils of a motor or a generator.
2. Related Background Art
Motors are employed in rotating parts of VTR's, still video equipments etc. Requirements for the motors are a uniform torque, compactization, improvement in maximum torque, improvement in torque-to-revolution characteristics etc.
For compactization of the motor, the Japanese Laid-Open Patents 57-186940, 57-186941 and 57-186942 disclose a motor coil of printed structure, obtained by etching a copper plate on an insulating substrate, instead of conventional coil composed of a conductor wound on an iron core.
The coil disclosed in the above-mentioned patent references is made very thin by etching the conductor constituting the coil (hereinafter called printed coil), and can provide a flat and compact motor. FIG. 1 shows an example of use of a motor with printed coils in a video cassette recorder.
In FIG. 1, coils 1, 1' formed on a substrate are fixed, through a fixed yoke 5', to a lower drum 6. When said coils, with a current therein, cross the magnetic flux from a magnet 4, a torque is obtained in a rotor equipped with said magnet. A yoke 5 of a magnetic material is provided for preventing the spreading of said magnetic flux and improving the efficiency. Said yoke is connected to an upper drum 3 through a rotary shaft 2 and functions as a rotor. The upper drum is equipped with magnetic heads 7, 7' for magnetoelectrical conversion, and a rotary transformer 8 is mounted on the upper and lower drums for transmitting the signals to or from said magnetic heads. In FIG. 1, the coil motor corresponds to a portion sandwiched between the yokes 5, 5'.
FIG. 3 shows the coil pattern of said motor. The pattern 10 is formed by etching a copper layer of about 100 microns in thickness adhered to a coil substrate 9. The coils thus prepared are superposed in plural layers, such as three or six layers, to obtain a coil unit. The width of coil wiring is represented by W, and the width of groove for forming the coil pattern is represented by .DELTA.W. The value of .DELTA.W should be made as small as possible in order to improve the motor efficiency by reducing the resistance of coils. In an example .DELTA.W=80 .mu.m while W=ca. 400 .mu.m, so that .DELTA.W/W.ltoreq.0.2. The groove width .DELTA.W is principally determined by the thickness of copper, while W is determined by the number of turns in a spiral coil and the magnitude of inverse electromotive force.
In recent years there has been remarkable progress in the superconducting transition temperature Tc of superconducting ceramics. Already in the ceramics of Y-Sc-Ba-Sr-Cu-M-O (M=metal) family, the superconducting state has been observed from an ultra low temperature state to a high temperature state. This material is applicable to a coil motor, and can achieve a maximum efficiency, utilizing the zero resistance. In fact the zero resistance in the current path drastically improves the resolution-torque characteristic, and provides a several times higher ability for maintaining a constant revolution.
However, the formation of coils with a superconducting material has resulted in various drawbacks which will be explained in the following in relation to FIG. 2, which is a circumferential cross-sectional view, in the vicinity of the center, of the coils shown in FIG. 3 and shows the shape parallel to the direction of rotation. There are shown a permanent magnet 4 magnetized in a direction indicated by an arrow; magnetic yokes 5, 5'; a coil substrate 9; and coil wires 10 perpendicular to the direction of current 11 indicates magnetic flux, while 12 is a symbol indicating the direction of current, and 13 is the groove of a width .DELTA.W.
Firstly, though it has been tried to reduce the width .DELTA.W of the groove, the magnetic flux is concentrated in the grooves 13 because of the perfect diamagnetism of the superconducting material, thus causing increase in the magnetic resistance of the magnet 4 and the yoke 5', decrease of magnetic flux, larger spreading thereof, and eventually decrease of the torque.
Secondly, the increase of the dimension of the current path increases the inductance, thus increasing the inverse electromotive force and giving a large load to the driving circuits
The above-mentioned drawbacks are not limited to a motor but are also encountered in a generator.
FIG. 4 shows a cylindrical coreless motor, of which coils are formed by winding a conductor in cylindrical shape as disclosed in the U.S. Pat. No. 4,327,304.
FIG. 5 is a partial cross-sectional view of the motor shown in FIG. 4. The magnetic flux emerging from the N-pole of a permanent magnet 12 crosses conductors 14, then passes through an external core 16 of a soft magnetic material, again crosses the conductors 14 and reaches the S-pole of the permanent magnet 12. A rotor 22 rotates in a direction 20 by appropriately regulating the direction 18 of the current in the conductors according to the direction of the magnetic flux.
A superconducting material, if employed in the coils of the cylindrical motor as shown in FIG. 4, leads to following drawbacks.
Because of the Meissner effect of the conductors made of the superconducting material, the magnetic flux 26 is unable to penetrate the conductors and assumes a distribution as shown in FIG. 6. As the magnetic flux has to go around the conductors 24, the length of the magnetic flux becomes longer, and the paths of the magnetic flux become narrower. Consequently the cross section of the magnetic flux is reduced to increase the reluctance, thus decreasing the amount of magnetic flux. In this manner the improvement in the efficiency obtained by the use of superconducting material, capable of reducing the resistance of conductors to zero, is eventually cancelled.